Relative pronoun's obsession

The professor said that relative pronouns are the most grammatical difficulty, which was left over behind the conquest for second language learners, and I totally agreed with that. Especially for Japanese, the style of modification in English is distinct from what Japanese normally construct the sentences.

We manage this long sentence: “the beautiful woman who lives in Japan” in Japanese as well. But if the sentence goes this long: “the beautiful woman who had dinner with me yesterday lives in Japan”, that would be hard to memorize about what was supposed to modify at the very beginning. I am not a grammarian, but something is significantly different from English on this point.

Many people, however, do not remember the differences between “which” and “,which”.

The book I learned today about the chapter of relative clauses says that the latter (with comma) gives us extra information about the person or thing, which means you already know which thing or person signify the “which”.

The example, “the beautiful woman who had dinner with me yesterday” does not tell who she is without the relative clauses because there are thousands of beautiful woman (including myself, perhaps) in the world. This is why we connect the clause with “who”.

If the sentence is as “She is Jane’s sister, who had dinner with me yesterday,” Jane’s sister is already identified as the woman whom you know in the sentence before at the relative clause. Then the following sentence after the relative pronoun “who” takes a role of extra information.

Both two are confusing, for it favors the grammar test. I sometimes forget “that” is not allowed to use in this case. After today, I will banish those mistakes from my writing forever.